An increasing amount of offshore petroleum exploration is being conducted at sites off the coasts of Alaska, California, Louisiana and Texas, and most recently in the North Sea. A drilling platform is typically installed on or over the sea floor at the desired site, after which a string of outside well conductors or pipes are lowered from the platform and driven into the sea floor to provide guidance and support for wells drilled into the hydrocarbon-bearing formations. Fluids produced from the formations are conducted to the surface through the casing and tubing set inside the well conductors.
Since it is not always possible to locate a drilling platform immediately above a formation, it is highly desirable in such situations to utilize well conductors having increased lateral reach or deviation from the vertical. For example, if the local soil conditions on the sea floor are such that mud slides could occur, a condition which is frequently encountered when drilling in the Gulf of Mexico near the mouth of the Mississippi River, such a site would not provide stable support for a platform and would therefore be unacceptable even though promising formations could lie underneath the area. More importantly, increasing the deviation or lateral reach of the well conductors enlarges the potential drainage area which can be explored from a single platform, and thus enhances the chances of making a productive discovery.
Various techniques have been developed for extending the lateral reach of well conductors. U.S. Pat. No. 3,451,493 to Storm discloses a drilling apparatus and method wherein straight well conductors are driven at an angle into the sea floor through guide tubes mounted in predetermined slanted orientation on the platform structure. The conductors thus extend outside the platform structure in the water, a condition which is undesirable since relatively long spans of unsupported well conductors may require construction of guides or supports external to the main framework of the platform. Such an arrangement is both costly and complicated, and may not provide sufficient lateral resistance to serve as a portion of the platform foundation. Moreover, special slant-type drilling rigs must be used instead of conventional straight rigs.
An alternative arrangement is to locate the well head for each straight conductor at the side of the platform such that the conductors criss cross in the water substantially inside the framework of the platform. This arrangement, while facilitating construction of guides and supports for the slanted conductors, consumes a large amount of deck space on the platform since the wellheads are not located together. In addition, this technique may result in an undesirable well conductor reach pattern.
More recently, curved conductors have been devised in an attempt to overcome some of the foregoing difficulties. For example, U.S. Pat. No. Re. 28,860 to Marshall et al discloses a technique wherein the conductor guides are mounted in an arc on the supporting framework of the platform. Precurved conductor pipe is inserted in the curved portion of the guides, from the bottom of the rig while onshore, and connected to straight conductor that extends to the rig. The pre-installed conductor pipe is then temporarily secured in place within the guides until after installation of the platform. Straight conductor sections thereafter connected and driven downwardly through the guides are deformed into an arcuate configuration which continues to build curvature as the conductors are extended downwardly and outwardly into the sea floor. Special slant-type drilling rigs are not necessary because the upper portion of each such well conductor is vertical.
Both of the foregoing techniques, however, suffer from the common disadvantage of being difficult to aim. Since the guides for the conductors are fixed to the platform in a predetermined arrangement, it will be appreciated that the platform must be oriented in the desired direction at the time of installation. The aiming direction of the conductors is thus determined at the time of installation of the platform, and cannot be changed thereafter except through reorientation of the platform. Drilling from such platforms is thus limited to the general directions of the conductors upon installation of the platforms.
A need has thus arisen for an improved method and apparatus for installing curved conductors which affords more flexibility in aiming the conductors and which is not dependent upon orientation of the offshore platform.